The invention relates to apparatus and methods for loading of quartz boats of semiconductor wafers into diffusion furnaces for processing at elevated temperatures, without generating excessive numbers of defect-causing particulates, and relates more particularly to cantilever diffusion tubes for carrying the wafer-loaded diffusion boats into diffusion furnaces without causing quartz-to-quartz abrasion, and relates still more particularly to methods and apparatus for effectuating loading and unloading of wafer boats into cantilever diffusion tubes, and yet more particularly to methods and apparatus for effecting the foregoing operations without sagging of the cantilever diffusion tube, even at extremely high temperatures in the furnace.
A cantilever tube system described in the above-mentioned Wollman application solves many of the problems associated with prior systems for loading diffusion furnace tubes, and particularly prior cantilever diffusion systems with rods that support loaded wafer boats in cantilever fashion within a diffusion furnace tube as an expedient for reducing generation of defect-causing particulates caused by quartz-to-quartz frition. The particular problems solved by the cantilever tube system described in the Wollman application are described in detail therein and, include avoiding excessive thermal shock to wafers being withdrawn from the hot zone of the diffusion furnace while nevertheless allowing relatively rapid withdrawal rates and use of far less nitrogen purging gas to isolate the wafers from premature exposure to atmospheric oxygen and thereby avoiding excess Q.sub.SS shifts. That cantilever tube system further isolates the semiconductor wafers, after they are withdrawn from the hot zone of the diffusion furnace and while they are cooling in the loading station, from particulates in the non-laminar air flow that usually exists in diffusion furnace loading stations. The cantilever tube system also greatly reduces the amount of cost and labor associated with the required frequent cleaning of diffusion furnace tubes, by confining nearly all contamination associated with reactor tube processes to the inside of the cantilever tube which can be quickly and easily removed and replaced by a clean one without excessive down time or inoperative time. Non-uniform gas flow caused by the presence of large cantilever rods of prior cantilever systems in the gas flow path is avoided by the system described in the Wollman application, and the high thermal mass and non-uniform temperature variations and resulting processing variations caused by prior cantilever loading systems are also avoided by the system described in the Wollman application.
The cantilever tube described in the Wollman application is loaded with wafer boats by passing the loaded wafer boats through a large side window in the wall of the diffusion tube. A close fitting quartz cover is positioned over the window after all boats have been loaded, before insertion of the cantilever tube into the hot zone of a diffusion furnace. While the technique of loading and unloading wafer boats through the side window in the diffusion tube is effective, it has become apparent that in some instances this technique is inconvenient and is not well suited to easily designed, low cost automated wafer loading systems.
Thus, there remains a need for a more convenient cantilever diffusion tube apparatus and technique for rapidly and inexpensively loading a plurality of diffusion boats loaded with semiconductor wafers into a cantilever tube.
Several embodiments of the invention described in the Wollman application provide wheels built into a cantilever diffusion tube for supporting the end and center portions of a cantilever diffusion tube to avoid sagging that would otherwise result from prolonged exposure of the cantilever diffusion tube to very high temperatures in the diffusion furnace tube. For example, for quartz cantilever tubes, temperatures in excess of approximately 1200.degree. Centigrade cause sagging of the cantilever tubes. Although the described technique is workable, there exists a need for a simpler approach to avoiding sagging of a cantilever tube exposed to exceedingly high temperatures in a diffusion furnace.
Some present cantilever systems, including the ones described in the above identified Wollman application, feed gas from the loading station side of the diffusion furnace, whereas conventional diffusion furnaces feed processing gases from the opposite end of the diffusion furnace. It would be helpful if there were a convenient, practical means of feeding gas into the cantilever tube of the above Wollman application with a gas feed connection to the pigtail of the diffusion tube.